The limited supply of fossil fuels and the environmental issues associated with their utilization has resulted in much effort put forth to promote renewable resources of energy. Switching to renewable fuels for energy will allow us to become carbon neutral by recycling carbon from plants and reduce carbon from dioxide emissions, which could potentially reduce global warming in future generations and generate new industries with exciting new technologies.
Lignocellulosic plant material is one of the most predominant natural compounds in the biosphere and a common source of feed ingredient for livestock as well as pulp and paper in the wood industry. Lignocellulosic plant material includes cellulose, non-cellulosic polysaccharides and lignin.
Lignin is an insoluble polymer which is a major component of lignocellulosic plants, for instance, constituting between 20-30% of the dry weight of wood. Lignin provides mechanical strength to plants and is primarily responsible for the rigidity of plant stems. Lignin also plays an important role in water transport, chemical resistance and disease resistance in lignocellulosic plants. Lignin is known in the art and is generally described in Hofrichter, M., Lignin, Humic Substances and Coal, Biopolymers, Vol. 1, Wiley-VCH, 2001.
Unfortunately, lignocellulosic material remains largely underutilized due to the difficulty of degrading lignin. For example, production of fermentable sugars is limited because lignin limits hydrolyzability of cellulose. The conversion of lignocellulosic material into ethanol involves hydrolysis of cellulose in the lignocellulosic materials to fermentable sugars, and then fermentation of the sugars to ethanol. Hydrolysis is typically catalyzed by cellulases, and the fermentation is carried out by yeasts or bacteria. Factors that affect the efficiency/cost of lignocellulose hydrolysis include: porosity (accessible surface area), cellulose crystallinity, and lignin content, as described in McMillan, J. D., Pretreatment of Lignocellulosic Biomass, in Enzymatic Conversion of Biomass for Fuels Production. 1994. p. 292-324. Note that each factor impacts accessibility of enzymes to the cellulosic substrate.
Overcoming the lignin barrier is a limiting factor in accessibility of cellulosic substrates, such as cellulose and hemicellulose, to hydrolytic enzymes in biomass conversion. Physical, chemical, and biological processes have been used to pre-treat lignocellulosic materials with the goals of breaking or disrupting the lignin network and/or reducing the crystallinity of cellulose. These processes alter existing structure, providing hydrolytic enzymes increased access to the substrate. Pretreatment is one of the most expensive steps in the conversion of lignocellulosic material to fermentable sugars. Additionally, the overall conversion is poor, due largely to the efficacious nature of lignin in limiting enzymatic access to cellulose and hemicellulose. Thus, facilitation of release of fermentable sugars from lignocellulosic material would be valuable in numerous industrial applications.
Lignin is also a principal impediment to digestibility of lignocellulosic forage crops. Higher lignin content is associated with decreased digestibility in cattle and other herbivores.
Biotechnological efforts to lower lignin content of plants have met with little success since lowered lignin content has been observed to have pleiotropic effects on plant development. These effects include leaf shape and texture, stunted growth, reduced pollen viability, and altered flower morphology, pigmentation, and collapsed vessels, for example as described in Casler, M. D., et al., Genetic Modification of Lignin Concentration Affects Fitness of Perennial Herbaceous Plants., Theor. Appl. Genet., 104:127-131, 2002; and Pedersen, J. F., et al., 2005, Impact of reduced lignin on plant fitness. Crop Sci. 45, 812-819.
Thus, limitations relating to lignin degradation are of central importance in biomass utilization. There is a continuing need for methods and compositions to produce plants characterized by improved processibility of plant lignocellulosic materials without undesirable side effects on development and growth of the plants.